Electrolytic silver recovery from used photographic solutions is a common way of extending the life of such solutions. An apparatus for the electrolytic recovery of silver from solutions containing silver is known from European patent application EPA 93200427.8 (Agfa-Gevaert NV) filed Feb. 16, 1993. The apparatus comprises an electrolytic cell having an anode and a cathode, and electrical power supply control means for controlling the supply of electrical power to the anode and the cathode. The control of the electrochemical process taking place at the anode and the cathode is important in the silver recovery process. If too high a potential difference is applied, side-reactions can occur, depending upon the nature of the silver-containing solution, leading to unwanted by-products, and adhesion of the silver to the cathode may be poor. If too low a potential difference is applied, de-silvering takes place only slowly. The de-silvering process proceeds by depositing silver upon the cathode. If the silver does not adhere strongly to the cathode, there is a risk that it will become detached therefrom, especially as the weight of silver deposited increases and especially in continuously operated cells which have a constant flow of electrolyte solution passing over the cathode. The detached silver may fall to the bottom of the cell where it eventually builds up to a level which may cause a short circuit between the anode and the cathode. Some detached silver may be flushed out of the cell with the electrolyte liquid. In either case the de-silvering of the solution is not optimally achieved.
The theoretical conditions required for optimum de-silvering depend upon a number of factors including the cathode potential, the concentration of silver in the silver-containing solution, the pH of the silver-containing solution (usually within the range of from 3.5 to 6.0), the composition of the seasoned fixer and the condition of the cathode. For a given fixer composition, de-silvering apparatus, pH and cathode condition, there is an ideal cathode potential, or narrow range of cathode potentials, which provides fast deposition, good adherence of the silver to the cathode and a low level of side-reactions. Outside these optimum conditions, these objectives may not reliably be met. As the de-silvering process continues, the concentration of silver in the silver-containing solution changes as silver is deposited and fresh solution is added, the pH of the solution is unknown or varies in an unpredictable manner and the condition of the cathode may change. It has not therefore been possible to set the electrolytic cell to the optimum de-silvering conditions and to maintain optimum conditions as the de-silvering continues.
There is a number of known methods of controlling the de-silvering process, referred to herein as (i) galvanostatic, (ii) constant potential difference, (iii) quasi-galvanostatic, (iv) quasi-potentiostatic and (v) potentiostatic control.
i) Galvanostatic Control
In galvanostatic control, a constant current flows through the cell while it is in operation. As the de-silvering progresses, the level of silver in the solution falls and the ohmic resistance between the anode and the cathode increases. It is therefore necessary to increase this potential difference in order to maintain a constant current. While the instrumentation required for this control is very simple, the method suffers from the fact that at high silver concentrations the potential difference is small and therefore de-silvering takes place only slowly, while at low silver concentrations the potential difference may be sufficiently high that undesirable side-reactions are liable to occur, adhesion of the silver to the cathode is bad and sulphidation of the cathode occurs.
ii) Constant potential Difference Control
In a constant potential difference control method, the potential difference between the anode and the cathode is kept constant as the de-silvering progresses. The disadvantage of this method is that the potential difference between the cathode and the solution is not controlled. The electrochemical reactions taking place at the cathode are therefore also uncontrolled, depending on a large number of factors such as the size of the anode, agitation in the neighbourhood of the anode, the presence or absence of components in the solution which can be oxidised and the ease with which they can be oxidised (e.g. SO.sub.3.sup.-- and S.sub.2 O.sub.3.sup.--), the ohmic potential drop in the cell and therefore also the cell geometry and current density, and the current through the cell.
iii) Quasi-Galvanostatic Control
In a quasi-galvanostatic control mode, the current through the electrolytic cell is kept at a constant target value, but this target value is itself adjusted from time to time, according to specific operational parameters of the de-silvering. One example of quasi-galvanostatic control is as follows: During operation of the cell, the silver content is determined by (external) analysis. The target current value is then adjusted to a specified value, depending on the silver content measured (high current for high silver contents, low current for lower silver content in the fixer), After some time, the analysis is repeated and the target current value is set according to the new silver content measured. The time interval between two successive silver analyses is determined by the speed of de-silvering. When de-silvering is very fast, e.g. 50% decrease of silver content in 60 min. the analysis may be necessary every 15 minutes. When de-silvering is very slow, e.g. 50% decrease of silver every 600 minutes, a new analysis of silver content is only required every 200 minutes. (The same may hold, of course, for very fast or slow addition of silver to the fixer to be de-silvered.) Generally speaking, in quasi-galvanostatic control, the frequency of updating of the target current value is determined by the time during which the operational parameters of the cell remain substantially constant. These operational parameters may be of electrochemical/chemical or mechanical nature.
The operational parameters include:
silver content in the fixer to be de-silvered, PA1 pH of the solution to be de-silvered, PA1 presence or absence of toxifying substances in the fixer, PA1 fixer flow rate through the electrolysis unit, PA1 conductivity of the electrolyte. An exhaustive list of possible operational parameters would include all factors which are important in the de-silvering process. In general, however, only a limited number of them are significantly variable in any given de-silvering situation. In many cases, only one factor is monitored (e.g. silver content). Other examples of quasi-galvanostatic control are: PA1 Estimation of the silver content in the fixer to be de-silvered by measurement of the rest potential of the electrolytic cell (rest potential of cathode versus anode) and adjustment of the cell current target value according to some predetermined relation between the measured rest potential and the current target value. The rest potential of the cell could be measured between the cathode and the anode, or between the cathode and some other electrode. That other electrode could for example be a reference electrode, for example a Ag/AgCl electrode. Alternatively, such a reference electrode could be a pH sensitive electrode. PA1 Estimation of the electrolyte flow rate through the electrolysis unit and adjustment of the target current value according to the flow rate. PA1 Estimation of the silver content of the solution to be de-silvered by analysing the response of the current to changes in the applied potentials (cathode/anode potential versus cathode/ anode/third electrode). PA1 Estimation of the silver content or detoxification potential by analysing the response of the current versus cathode potential (or any other potential difference between cathode/anode/third electrode) and adjustment of the current. PA1 U=U.sub.O, PA1 (i) supplying silver-containing solution to the electrolytic cell to immerse the anode, cathode and reference electrode therein; and PA1 (ii) applying electrical power to the anode and the cathode such that the cathode potential relative to the reference electrode, lies within a de-silvering range to cause silver to be deposited on said cathode; PA1 characterised in that operation of the cell is controlled potentiostatically while the current exceeds a certain threshold value and in that the current is controlled galvanostatically at that threshold (minimum) value in circumstances in which the potentiostatic control would result in a current below the threshold value.
iv) Quasi-Potentiostatic Control
In quasi-galvanostatic control, it is the target current which is adjusted as a function of one or more operational parameters of the de-silvering. In quasi-potentiostatic de-silvering, a target potential difference is adjusted as a function of one or more operational parameters of the de-silvering. This target potential difference may be any accessible potential difference in the electrolysis cell (potential differences between cathode, anode, reference electrodes, pH sensitive electrodes, other electrodes). An important example of quasi-potentiostatic control is a control algorithm according to which the anode-cathode potential difference is kept constant at a value which is determined by the current flowing through the cell.
v) Potentiostatic Control
In potentiostatic control, a reference electrode is included in the electrolytic cell and the potential difference between the cathode and the reference electrode is kept constant. This allows complete control over the cathode potential. This method of operation is therefore widely preferred, since it is the cathode potential which determines electrochemical reactions which take place in a fixer of a certain composition. By using a reference electrode, the influence of the anode potential (and largely also the ohmic potential contributions) are excluded. This enables the initial cathode potential to be set at a level where bad silver adhesion, side-reactions and sulphidation of the cathode can be avoided, independently of the anode potential. The use of a reference electrode makes the equipment more reliable, since factors such as the current density at the anode, the surface state of the anode, over-potential at the anode (caused by changes in solution composition), and ohmic potential drops no longer influence the cathode potential. As the de-silvering process continues and silver is removed from the solution, the current through the cell falls while the potential difference between the cathode and the reference electrode is maintained at a fixed level. When fresh solution with a higher silver content is subsequently added, the current through the cell will normally increase and silver continues to be deposited on the cathode.
These five basic de-silvering control methods referred to above form the basic principles underlying a number of possible actual control mechanisms which are used in commercial equipment. In commercial equipment, the control method may be a hybrid of a number of the control methods referred to. For example, many commercial applications use the combination of constant anode-cathode potential difference and quasi-potentiostatic control, as the applied cathode-anode potential is kept constant to a predetermined level which is set according to the current measured through the cell. The advantage of potentiostatic control has long been recognised (see for example French patent FR 1357177 (Bayer) and it is also used in commercial equipment (e.g. ECOSYS F08, and ECOMIX from Agfa-Gevaert NV).
Another interesting patent U.S. Pat. No. 5,310,466 (granted to John La Riviere et al.) describes a quasi-potentiostatic control mechanism. More in particular, it describes a specific case of a quasi-potentiostatic control mechanism according to which the anode-cathode potential difference is kept constant at a value which is determined by the current flowing through the cell.
In practice, it is sometimes observed that, although the silver content of the bath to be de-silvered is high (for example Ag&gt;3 g/L) and the de-silvering apparatus as such is in good working order, no silver becomes deposited on the cathode. This effect is attributed to "cathode poisoning". Poisoning occurs when components present in the solution block the cathode reduction process. Not all cases of cathode poisoning are understood, but certain components which are present in the developer or which are flushed out of the film may be the cause. A number of photographic stabilisers exhibit this effect, such as phenyl mercapto tetrazol. In European Patent Application No. 95.201.954.5 (Agfa-Gevaert NV), we have proposed a process for de-silvering a silver-containing solution in which, whereas the potential difference between the cathode and a reference electrode normally lies within a de-silvering range to cause silver to be deposited on said cathode, the cathode potential is at least periodically reduced (i.e. made more negative) as the de-silvering process continues.
In such a periodic control method, the cell is firstly operated under potentiostatic conditions. After a given period of time, the cathode potential is decreased to a predetermined level. Thus, when the solution is placed in the cell, and the apparatus is switched on, the cell current is rather high and the cathode potential is set at a first level. As the de-silvering process continues and the level of silver in the solution falls, the cell current falls. During this time the cathode potential is maintained at its first level. After a given period of time however, the cathode potential is adjusted to a lower (i.e. more negative) level. The cathode potential is held at this lower level for a given period of time, referred to as a detoxification period, after which it is returned to the first level. Such process may include a plurality of de-silvering steps interposed by relatively brief detoxifying steps. In another control method which may be operated continuously, it was proposed that the cathode potential be reduced according to a predetermined relationship between the cathode potential and the current flowing through the cell as the de-silvering process takes place, independently of any changes in the anode potential. The predetermined relationship between the cathode potential and the cell current is of a form whereby the cathode potential is lower (i.e. more negative) as the cell current falls. In simplified form this relationship may be expressed as: EQU U=U.sub.O +k(I-I.sub.max)
where U is the cathode potential when the cell current is I, U.sub.O is the cathode potential when the cell current is at its maximum I.sub.max, and k is a positive non-zero coefficient, which in the simplest case is a constant. Note that if k were zero, this relationship would reduce to
i.e. the potentiostatic control method.
In such a relationship, k is so chosen that the minimum cathode potential (which occurs when I=0) i.e. U.sub.O -k.I.sub.max, is not so low that undesirable side-reactions occur to a significant extent. In the case of cathode poisoning, it may be advisable to chose larger values for k. This k is also chosen so as not to fully compensate for the fall in current, i.e. the cell current will continue to fall as the de-silvering continues. If k were so chosen as to fully compensate for the fall in cell current, the cell current I would remain constant, corresponding to the galvanostatic control method.
This control method may be carried out so that the cathode potential is continuously adjusted to a level determined by the cell current, in accordance with the predetermined relationship. Thus for example, when the solution is placed in the cell, the cell current is high and the cathode potential is at a first level. As the de-silvering process continues and the level of silver in the solution falls, the cathode potential is adjusted to lower (i.e. more negative) levels determined by the cell current, in accordance with the predetermined relationship. When, for example, the silver levels rise, the cell current rises and the cathode potential is adjusted to higher (i.e. less negative) levels determined by the cell current, in accordance with the predetermined relationship and the control sequence is repeated.
That proposal was based upon the discovery that the effects of poor silver adhesion and cathode poisoning during the de-silvering step could be substantially overcome by applying a lower cathode potential during part of the de-silvering process, so that in the next de-silvering step the efficiency of the process substantially returned and was maintained for a number of further de-silvering steps.
While it is known that the application of a lower cathode potential may lead to side-reactions and the generation of unwanted by-products, it was thought that by limiting the cathode potential to a level determined by the cell current, or by limiting the total charge which is passed through the electrolysis cell, the effect of such side-reactions, such as for example the reduction of sulphite at the cathode surface, over the whole de-silvering process would be minimised.
An even better way to circumvent silver adhesion problems and cathode poisoning problems is the use of a galvano-dynamic control algorithm, as described in European Patent Application No 95.201.953.7 (Agfa-Gevaert NV). In this mode of operation, the diffusion limitation current is estimated by applying a more negative potential to the cathode. Afterwards, the current is galvanostatically controlled to a value which is a certain fraction of this estimated limitation current density. By doing this, one avoids depositing substantial amounts of silver while current limitation conditions subsist, thus avoiding bad silver adhesion.
Moreover, by the use of a galvanostatic control routine, cathode poisoning is largely circumvented, since the cathode potential will automatically be made negative enough to avoid cathode poisoning. In comparison with the automatic decrease of cathode potential as the current drops, as described in European Patent Application No 95.201.954.5, the advantage of the galvano-dynamic control algorithm is that it works better when de-silvering conditions, such as the pH of the solution, vary.
The galvano-dynamic current control algorithm performs well when the cathode detoxification potential (the cathode potential at which the poisoning is overcome by the more negative cathode potential) is not less negative than the potential used to estimate the diffusion limitation current density (e.g. -530 mV versus a glass electrode). If the detoxification potential is more negative (e.g. -590 mV versus a glass electrode), no detoxification will take place during the measurement of the limitation current density, and the resulting currents will be very low, resulting in a very slow de-silvering.
Reduction of the cathode potential to more negative values (e.g. -600 mV versus glass electrode) for determination of the limitation current may not be advisable, since in this case, side reactions such as sulphite reduction and eventually also sulphidation of the cathode may occur. This would result in, among other unwanted effects, an over-estimation of the silver limitation current, and consequently bad silver adhesion.
Thus, the galvano-dynamic control algorithm is not ideally suited for de-silvering fixers of low pH, where onset of sulphite reduction occurs very rapidly because the cathode potential at which sulphite reduction starts to occur is pH dependent. It is also less suited in situations where very severe cathode poisoning occurs. It was discovered that e.g. in the field of X-ray processing, in order to draw current through the cell and deposit silver on the cathode, it may be necessary to increase the potential difference between the cathode and the electrolyte to such an extent that in the absence of any poisoning, the majority of the current drawn would be due to sulphidation rather than silver reduction. Thus the potential required to be applied to the cathode to overcome the poisoning may be so great that in the absence of any such poisoning, it would engender significant side-reactions, and in the result it would be impossible to tell how much of the current flowing was due to desirable deposition of silver and how much to such undesirable side-reactions.
In difference to e.g. U.S. Pat. No. 5,310,466, the present patent application comprises a control mechanism wherein a truely potentiostatic control (keeping the potential of the cathode constant with respect to a reference electrode) is combined with a type of galvanostatic control. By doing so, we get a system which has both the advantages of a galvanostatic control and the advantages of a potentiostatic control. Thereabove, the present invention does not have the disadvantages of a quasi-potentiostatic control device such as the one described in U.S. Pat. No. 5,310,466, and is less susceptible to cathode poisoning.